Major General Chip Chapman told Sky News: “It seems to be this bomb either malfunctioned or it’s not TATP (triacetone triperoxide), the explosives used by groups like ISIS.

“It’s more like the Boston bomb. It’s almost like a pressure cooker device.”

The 2013 Boston Marathon bombing featured two homemade explosive devices that were detonated 12 seconds apart from one another, killing three and injuring 280.

The pair responsible for the attack used pressure cooker devices – containers packed with explosives and loaded with nails and ball bearings to inflict maximum damage.

Maj Gen Chapman added: “This doesn’t look like a high-end explosive from ISIS such as TATP or, if it was, it failed significantly, the booster or detonator didn’t go off.

“It’s not a high explosive that functioned, because the blast and shock wave would have killed multiple people.”

Similarities were also drawn between the device and those which failed to explode on the London Underground on July 21, 2005. However, authorities have not yet confirmed what materials the bucket contained.

Hannah Bryce, from thinktank Chatham House, told The Sun Online it looked like it had “only partially detonated”.

She added: “These are often unreliable devices which may explain why it did not fully detonate.

“Looking at the size of the bucket it could have held a large amount of explosive material which could have had an even more devastating impact than it did.

“Whilst not detracting from the gravity of this incident there is some reassurance to be had from the seeming lack of access to commercially manufactured explosive materials.

“Whilst HME can be equally as damaging, they tend to be less powerful than bombs made with commercial explosives.”

Terrorism analyst Magnus Ranstorp, from the Swedish Defense University, said Londoners were “really lucky with this one, it could have really become much worse”.

He noted the bomber chose to conceal the device in a bucket and a plastic shopping bag rather than a backpack and appeared “hastily put together – probably not very well mixed together”.

Other security experts told The Sun Online the flaming bucket photographed in the train carriage may have malfunctioned on its way to a larger station.

It appeared to have been constructed out of flammable materials, a plastic bucket and fairy lights. The BBC reports it also featured a timer.

Security expert Will Geddes told The Sun Online: “My feeling is that it’s a premature detonation. I know both Parsons Green and District Line very well. It’s an ideal commuter time.

BTP confirm that anti-terror police are now in control of investigation into Parsons Green Tube explosion

"It was a premature detonation because it doesn't sit inside the type of profile that terrorists want for a detonation... Notting Hill or Paddington Station - then you've got a much more identifiable location for the world to pick up on.

"The device is really basic. I don't see a considerable amount of scorched surfaces around the device itself.

"It looks a firework bucket - (the explosion) actually went up and not outwards. I would say it's an ill-conceived device. But there were very significant burns - there was likely to be some kind of toxic element to it."

Raffaello Pantucci, International Centre for the Study of Radicalization at King's College, described it as a "very rudimentary device".

"It looks like a fairly rudimentary device. It looks like someone has tried to build something out of explosive materials in a plastic container and shrouded it in a plastic bag on the outside.

BIRMINGHAM'S FAIRY LIGHTS BOMB PLOT

Devices similar to that seen on the Parsons Green train carriage have been created by terrorists previously.

In May, Zahid Hussain, 29, was convicted of preparing for terror at Birmingham crown court.

His plot featured a pressure cooker nail bomb using a fairy lights detonator.

Hussain's trial was told he wrongly believed his non-viable "bomb" - packed with shrapnel - was capable of causing devastation.

The 29-year-old - who was captured on CCTV clambering into a storm drain near a high- speed rail line - was arrested in August 2015 after being seen "patrolling" the streets near his home in Naseby Road, Alum Rock.

Jurors at Birmingham Crown Court deliberated over two days before convicting Hussain of preparation of terrorist acts.

"The device didn't actually do what it was hoped to do, which leads me to suspect it was a fairly amateur bomb maker.

"There's been a history of this stuff - this one doesn't look like a very professional one but you never know. Making a bomb is not that hard these days.

"It didn't really blow up. The fact that it is just sitting there, the fact that it's just a bucket like that - a bucket well wrapped in material.

"It could just be the detonator went off and the rest of it didn't."

He added that he would expect to see remnants of chemicals surrounding the bucket if such materials were present in its construction.

"Anyone who is trying to set off a bomb is trying to do maximum damage but what they intended, I have no idea."

Armed police take up positions at Parsons Green Tube station after terror attack